To calculate the cost of renting the hall for 70 students attending the prom using the formula \( c(n) = 500 + 4n \), we will substitute \( n \) with 70.
\[ c(70) = 500 + 4(70) \]
Calculating \( 4 \times 70 \):
\[ 4(70) = 280 \]
Now, substituting this back into the equation:
\[ c(70) = 500 + 280 \]
Finally, add the two values:
\[ c(70) = 780 \]
Therefore, the cost of renting the hall for 70 students is $780.